Field of the Invention
This invention relates generally to the field of optical concentration and diffusion, such as solar concentration in solar energy systems, and more specifically to waveguide-based optical concentration/diffusion systems.
Description of the Related Art
In solar energy systems, solar concentration is a way to reduce the overall photovoltaic (PV) surface area that is necessary to produce a given amount of energy. The general principle of solar concentration is the collection of light over a relatively large surface area and transmitting it to a comparatively smaller PV surface. Assuming that the optical system is cheaper than a PV cell having the larger surface area, the reduction in size of the PV surface allows the use of cells having higher efficiencies without increasing cost.
In a solar concentrator, the ratio between the size of the entrance aperture and the size of the exit aperture is referred to as the geometric concentration factor C, while the ratio between the total optical energy reaching the exit aperture relative to that which is incident on the entrance aperture is called the optical efficiency. In practice, the concentration factor is limited by the field of view (FOV) of the optical system. Thus, if φ is the FOV (i.e., the acceptance angle) of the solar concentrator, the maximum concentration factor Cmax is given by the following equation:
      C    max    =            n      2                      sin        2            ⁢      φ      where n is the index of refraction of the medium into which the light is coupled.
Many of the disadvantages of existing solar concentrators are related to the limitation imposed by the FOV of the system. Planar systems like fluorescent concentrators tend to not be very efficient, and then can only attain small concentration factors. Static imaging or non-imaging concentrators can also be used. Since they are stationary while the sun changes position, they cannot capture much of the incoming solar energy during the year. Indeed, from the foregoing equation, it is evident that a stationary solar concentrator will have a FOV restricted to certain parts of the sky and that, to increase C, the concentrator would have to be installed on a sun tracker.
Sun trackers are known in the art, and work by changing the orientation of a solar collector to increase the overall time during which incoming sunlight remains within its FOV. Two general types of sun trackers are “single-axis” systems, which have movement along one axis of rotation, and “dual-axis” systems, which have two perpendicular axes of rotation. Single-axis tracking systems collect much more light than static systems and reach a higher concentration factor. Dual-axis tracking systems collect more light annually than single-axis systems, and reach an even higher concentration factor, but are more costly to implement. Such systems also require heavy, two-axis motorized trackers that cannot be installed in many locations.
One of the known types of solar concentrator available is a waveguide solar concentrator (WSC). A conventional WSC is shown schematically in FIG. 1. The WSC shown in the figure uses a light focusing element 12, such as an objective lens, and focuses it to a focal point within a waveguide 14, which is separated from the focusing element 12 by air or by any material having a different refractive index compared to the waveguide. Located at the focal point is a reflective component 16 that redirects light received from the focusing element within the waveguide at an angle that results in total internal reflection (TIR) of the light within the waveguide. The light thereby collected then travels along the waveguide and finally exits at an end having a relatively small area, and is incident upon an appropriate PV element.
In a typical configuration, a plurality of focusing elements 12 are provided along a top of the concentrator, each having a focal point at a different reflective component 16. The reflective components all redirect the collected light in the same direction along the waveguide such that the overall light energy exiting the end of the waveguide is increased. However, due the presence of many of the reflective elements, undesired reflections of light propagating within the waveguide may result, causing light previously undergoing TIR to be redirected at an angle that allows it to escape from the waveguide.
The reflective components 16 of the waveguide may take different forms. Typically, a structure such as a grating, or a convex or concave prism is used to redirect light in the desired direction. These structures are effective at coupling light into the waveguide but, as mentioned above, losses arise due to the interaction of light rays in the waveguide with these structures, which results in unintended reflections.
A WSC using cylindrical lenses as focusing elements is of interest because it is lightweight and compact, relatively inexpensive to fabricate, and because it can function with only one axis of sun tracking. However, because of rays lost during propagation within the waveguide, the optical efficiency is comparatively low for a given waveguide length. Moreover, there is a reduced optical efficiency due to Fresnel reflections at various air/material interfaces along the light path, such as surfaces 17 and 18 shown in FIG. 1. Finally, the relative alignment of focusing elements 12 and waveguide 14 requires supplemental manufacturing steps to ensure that the reflective elements 16 are positioned at the focal point of their corresponding focusing elements 12.